1. Field of the Invention
The present invention relates to a flow control valve. More particularly, the present invention relates to a flow control valve for controlling the oil pressure applied to a hydraulic actuator.
2. Description of the Related Art
A forklift for driving a fork that holds a load by using oil pressure has been well known. The forklift includes a lift cylinder for driving the fork that holds a load along with a flow control valve. FIG. 1 is a schematic view showing the conventional flow control valve. The flow control valve 101 includes a direction switching valve 102 and a check valve 103. The flow control valve 101 further includes a plurality of lines for guiding hydraulic oil to transmit oil pressure. The plurality of lines is composed of a pump pressure line 111, a pump pressure line 112, a load pressure line 113 and a drain line 115.
The pump pressure line 111 connects the direction switching valve 102 to a pump not shown and leads hydraulic oil supplied by the pump. The pump pressure line 112 connects a check valve 103 to the direction switching valve 102. The load pressure line 113 connects the check valve 103, the lift cylinder 104 and the direction switching valve 102. The drain line 115 connects the direction switching valve 102 to a tank 106 and the oil pressure of the drain line 115 is substantially zero (0).
The check valve 103 prevents hydraulic oil from flowing from the load pressure line 113 to the pump pressure line 112. That is, the check valve 103 connects the pump pressure line 112 to the load pressure line 113 when the oil pressure of the pump pressure line 112 is larger than that of the load pressure line 113, and does not connect the pump pressure line 112 to the load pressure line 113 when the oil pressure of the load pressure line 113 is larger than that of the pump pressure line 112.
The lift cylinder 104 is an actuator for lifting and lowering the fork of the forklift. That is, the lift cylinder 104 lifts the fork of the forklift when hydraulic oil is supplied from the load pressure line 113 and lowers the fork of the forklift when hydraulic oil is discharged into the load pressure line 113. At this time, the oil pressure of the load pressure line 113 varies depending on the weight of a load held by the fork of the forklift and becomes larger as the load is heavier.
The direction switching valve 102 can occupy one of a neutral position, a meter-in position and a meter-out position. That is, operated by the user, the direction switching valve 102 is switched from the neutral position to the meter-in position, from the neutral position to the meter-out position, from the meter-in position to the neutral position and from the meter-out position to the neutral position.
At the meter-in position, the direction switching valve 102 connects the pump pressure line 111 to the pump pressure line 112, closes the load pressure line 113 and closes the drain line 115. At the meter-out position, the direction switching valve 102 closes the pump pressure line 111, closes the pump pressure line 112 and connects the load pressure line 113 to the drain line 115. At the neutral position, the direction switching valve 102 closes the pump pressure line 111, closes the pump pressure line 112, closes the load pressure line 113 and closes the drain line 115.
The tank 106 stores hydraulic oil flowing through the drain line 115 therein. The hydraulic oil stored in the tank 106 is supplied to the pump pressure line 111 by a pump not shown.
Operations of the flow control valve 101 include a meter-in operation, a neutral operation and a meter-out operation. The meter-in operation is an operation performed when the direction switching valve 102 is switched from the neutral position to the meter-in position by means of the user's operation. The neutral operation is an operation performed when the direction switching valve 102 is switched from the meter-in position or the meter-out position to the neutral position by means of the user's operation. The meter-out operation is an operation performed when the direction switching valve 102 is switched from the neutral position to the meter-out position by means of the user's operation.
In the meter-in operation, hydraulic oil is supplied from the pump pressure line 111 to the lift cylinder 104 through the direction switching valve 102, the pump pressure line 112, the check valve 103 and the load pressure line 113. When the hydraulic oil is supplied, the lift cylinder 104 lifts the fork.
In the neutral operation, since the switching valve 102 closes connection between the pump pressure line 111 and the pump pressure line 112 and between the load pressure line 113 and the drain line 115, no hydraulic oil of the lift cylinder 104 is supplied or discharged and thus lifting or lowering of the fork is stopped. At this time, the load pressure varies depending on a load held by the fork of the forklift and becomes larger as the load is heavier.
In the meter-out operation, hydraulic oil is discharged from the lift cylinder 104 to the drain line 115 through the load pressure line 113 and the direction switching line 102. When the hydraulic oil is discharged, the lift cylinder 104 lowers the fork.
Even when the operation quantity of the direction switching valve 102 is identical, the higher the oil pressure of the load pressure line 113 is, the higher the hydraulic oil flows from the load pressure line 113 to the drain line 115. That is, in the forklift to which the flow control valve 101 is applied, even with the same operation quantity, the heavier the held load is, the faster the folk is lowered. A forklift with a fork having high operability has been desired.
In conjunction with the above description, Japanese Laid-Open Patent Application JP-A-Heisei, 08-100804 discloses a pressure compensating valve which only varies a set pressure of a relief valve without exchanging a piston, etc. The pressure compensating valve includes: a valve for opening and closing an inlet port and an outlet port; a piston for pressing the valve in the closing direction with a load pressure within a pressure chamber; an intermediate pressure chamber connected to the inlet port through a small cavity for pressing the valve in the closing direction; and a variable set pressure relief valve for relieving pressure oil in the intermediate pressure chamber to the outlet port through the small cavity.